The HMT 100 is part of Vaisala’s humidity transmitter line for fixed installations, utilizing Vaisala’s HUMICAP® humidity sensor technology.
Figure 1: Vaisala HMT 100
As the HMT 100 is a humidity and temperature transmitter, it relies on internal sensors to take the measurements. For humidity, the HMT 100 utilizes the Vaisala HUMICAP® 180, a capacitive thin-film polymer sensor. The thin-film polymer either absorbs or releases water vapor as the relative humidity of the ambient air rises or drops. The dielectric properties of the polymer film depend on the amount of water contained in it: as the relative humidity changes, the dielectric properties of the film change, and so the capacitance of the sensor changes. The electronics of the instrument convert the capacitance into a current or voltage output which indicates the humidity.
The HMT 100 utilizes a Platinum Resistance Temperature Detector (RTD) for temperature measurements. The resistance of an RTD increases as the temperature of the surrounding environment increases. The electronics of the instrument measure the resistance of the sensor, converting it into a second current or voltage output proportional to the temperature.
A number of variations exist for the HMT 100 model. These variations consist of different input and output power options, as well as output quantities. With regard to input and output power options, it is possible to choose a model powered with 10-35 VDC or 24 VAC input voltage and an analog output signal of either 4-20 mA, 0-1 V, 0-5 V, or 0-10 V. Additionally, the HMT 100 can be configured to output either one or two quantities, in the following combinations: relative humidity, dewpoint temperature, relative humidity and temperature, dewpoint temperature and temperature. These operating configurations and specifications are set during order time and cannot be changed by the user.
As is, the HMT 100 can be installed to work with the NI WSN indoors or outdoors. It features an IP65 classification housing. Optionally available within the HMT 100 are an aluminum wall installation plate, a duct installation kit, a weather shield for the transmitter, and a rain / solar radiation kit for the probe.